1. Field of the Invention
The invention relates to the field of amplifiers and in particular to temperature compensation of an amplifier stage.
2. Description of Related Art
Metal oxide semiconductor field effect transistors (MOSFETs) as well as other kinds of transistors are temperature dependent, and the current through a MOS transistor decreases as the temperature increases. This implies that the power output of an integrated circuit comprising such transistors also decreases as the temperature increases. The gain of such integrated circuits therefore varies a great deal in dependence on the operating temperature; the gain is reduced as the temperature increases. This is naturally also true for CMOS (Complementary MOS) transistors comprising p-type and n-type MOSFETs.
For amplifier stages, such as for example output drivers, a constant gain is desired to ensure constant power output and high linearity of the amplifier stage.
To provide such constant gain, one solution is to optimize the gain and linearity for the worst-case scenario, i.e. the worst-case temperatures and allow performance penalty or regulation loop and trimming to ensure the system performance in the presence of gain losses. For example, adding a circuit comprising one or more resistors and diodes can compensate for the temperature dependence of an amplifier. The characteristics of these resistors and diodes are chosen so that the mentioned worst-case temperatures can be handled.
Another commonly used solution is to design a device comprising an amplifier to have a programmable current, wherein the supply current is programmable over a certain range. In such a solution, a certain predetermined current is pre-programmed for use at a certain temperature. However, a disadvantage of using a programmable current amplifier is that the manufacturing costs are increased due to the additional and complicated manufacturing steps required. Further, the accuracy of such solution is not always adequate, as the current can only be programmed in certain discrete steps.
From the above, it is clear that it would be desirable to provide an improved amplifier stage, which is less temperature sensitive. In particular, it would be desirable to provide an improved temperature compensating circuit for an amplifier stage.